This application claims priority to South Korean Application No. 2001-24607 filed May 7, 2001.
1. Field of the Invention
The present invention relates to a method of fabricating electrostatic vertical and torsional actuators using the MEMS (Micro Electro Mechanical System) fabrication method and, more particularly, to a method of fabricating electrostatic vertical and torsional actuators using one homogenous single-crystalline silicon wafer, which is achieved by improving the prior art of using a number of bonded silicon wafers or a single/double SOI (Silicon-On-Insulator) wafer.
2. Description of the Related Art
The MEMS fabrication method relates to fabricating single devices and integrated systems of the micrometer dimension on a silicon wafer. The MEMS fabrication method has its basis on the method of fabricating silicon semiconductor electronic devices. The representative systems which are fabricated using the MEMS fabrication method are accelerometers which measure acceleration of moving objects, angular rate gyroscopes which measure angular velocity of rotating objects, and optical switches which control optical paths.
Recently, the research on the technology for fabricating high-aspect-ratio microstructures using single-crystalline silicon has been actively conducted for performing the improvement of various MEMS devices and systems. A microstructure of high-aspect-ratio has a large capacitance, which makes it possible to fabricate a sensor requiring a high precision level or an actuator which generates a strong force. In addition, a single-crystalline silicon structure does not have the problems that occur frequently, when deposited materials such as poly-crystalline silicon, or bonded materials such as SOI wafers and bonded wafers are used as the structural material. Such problems include the residual stress due to the use of deposited structural films, the residual stress from bonding different wafers, and the gradient of the residual stresses.
The surface/bulk micromachining (SBM) technology is known in the related art for fabricating a silicon microstructure of high-aspect-ratio. (Korean Patent No. 300,002 and U.S. Pat. No. 6,150,275 granted to the present applicant). According to the SBM technology, a released structure is fabricated from one homogeneous single-crystalline silicon wafer. Thus, there is no problem of residual stress or stress gradient and there is no need to stack and bond wafers as in the process of SOI or SOG (Silicon-on-Glass). However, the SBM technology is limited to fabricating devices with one planar structural layer.
The present invention relates to a method of fabricating electrostatic vertical and torsional actuators, that require two or more planar structural layers, using one homogeneous silicon wafer, by improving the above-described surface/bulk micromachining (SBM) technology. Electrostatic vertical and torsional actuators require at least two structural layers that are offset vertical layers, or two structural layers defined in different planes, in order to generate vertical forces or torquing moment. This configuration of fabricating two planar structural layers is difficult with the SBM fabrication technology.
The related art regarding a vertical or torsional actuator using the MEMS method has been introduced by Selvakumar et al. [A Selvakumar, K. Najafi, W. H. Juan and S. Pang, xe2x80x9cVertical comb array microactuators,xe2x80x9d IEEE Proc. MEMS ""95, 43-48, 1995]. This related art is about the method of fabricating a vertical micro actuator using a vertical comb array structure, by forming a trench in a silicon wafer and refilling the formed trench by the poly-crystalline silicon deposition process.
An other related art about a vertical and torsional actuator using the MEMS method has been introduced by Yeh et al. [J. A. Yeh, H. Jiang, and N. C. Tien, xe2x80x9cIntegrated polysilicon and DRIE bulk silicon micromachining for an electrostatic torsional actuator,xe2x80x9d IEEE/ASME J. Microelectromech. Sys., 8(4), 456-465, December 1999]. This related art is about the method of fabricating an electrostatic torsional actuator by using a poly-crystalline silicon film and DRIE (Deep Reactive Ion Etching) bulk silicon micromachined parts of a SOI wafer.
In the above-mentioned related arts, deposited poly-crystalline silicon films are used as moving structures, which are defined in a different plane than fixed structures.
Another related art has been introduced by Jeong et al. [H. Jeong, J. Choi, K. Y. Kim, K. B. Lee, J. U. Jeon and Y. E. Pak, xe2x80x9cMilli-scale mirror actuator with bulk micromachined vertical combs,xe2x80x9d IEEE Proc. Transducers ""99, 1006-1010, 1999]. This related art is about a millimeter scale secondary mirror actuator with DRIE bulk micromachined vertical combs fabricated by stacking and bonding a number of silicon and glass wafers.
Conant et al. [R. A. Conant, J. T. Nee, K. Y. Lau, and R. S. Muller, xe2x80x9cA flat high-frequency scanning micromirror,xe2x80x9d Proc. Hilton Head 2000, 6-13, 2000] fabricated a torsional mirror by forming of lower electrodes on a substrate silicon wafer, bonding another cap silicon wafer on the substrate silicon wafer, grinding the two silicon wafers, and then, forming upper electrodes on the cap silicon wafer.
Generally, it is very difficult to fabricate an actuator that moves vertically or torsionally on one single-crystalline silicon wafer. Accordingly, as in the above-mentioned related arts, a number of stacked-silicon wafers or single/double SOI wafers or combining of these wafers with deposited poly-crystalline silicon films have inevitably been used. Also, separate photomasks are generally used for fabricating the upper electrode and the lower electrode of the actuator. Thus, it is difficult to align the gap between the upper electrode and the lower electrode to a precise dimension, due to the alignment errors generated in aligning different photomasks.
The present invention, to resolve the above-described problems of the related art, has its purpose in providing a method of fabricating an electrostatic vertical and torsional actuator using one homogeneous silicon wafer, by improving the surface/bulk micromachining (SBM) method.
In order to achieve the above-mentioned purpose, the present invention fabricates an electrostatic combs with a vertical offset and horizontal gap using one single-crystalline silicon wafer(111) by improving the SBM technology, which is achieved by using two steps of silicon etch mask patterning and four steps of silicon etching process, wherein the etching depth may be controlled to achieve arbitrary comb dimensions and the vertical offset distance between the moving combs and the fixed combs, as well as the vertical space between the two said sets of combs.
More specifically, the method according to the present invention comprises the following steps:
step (a) of depositing silicon etch mask 1(112) on a silicon wafer and patterning it by the first photolithography process, thereby determining all the planar dimensions of the vertical or torsional actuator from the same photomask;
step (b) of depositing silicon etch mask 2(113) on top of the already patterned silicon etch mask 1(112) of step (a), and patterning it by the second photolithography process, making the upper comb electrodes of the vertical or torsional actuator to be completely covered by the silicon etch mask 2(113), while parts of silicon substrate is exposed;
step (c) of performing the first silicon etching by an amount of t1, during which the silicon etch mask 2(113) and the parts of the silicon etch mask 1(112) that is not covered with the silicon etch mask 2(113) are also simultaneously etched, such that the silicon etch mask 1(112) has different thicknesses between the parts that are covered with the silicon etch mask 2(113) and the parts that are not covered with the silicon etch mask 2(113);
step (d) of removing the silicon etch mask 2(113);
step (e) of determining the thickness of the upper electrode(118) by performing the second silicon etching by an amount of t2 in the state where the silicon etch mask 2(113) has been removed at the step (d);
step (f) of depositing a wall protection layer(114) on the vertical sidewalls of the structure which has been formed through the step (e);
step (g) of determining the vertical gap between the substrate and the lower electrode(119) by performing the third silicon etching by an amount of t3 after the step (f);
step (h) of laterally underetching silicon parts that are not covered with the protection layer and that are exposed in the step (g), in the horizontal direction using aqueous alkaline etchant, such that the body of the structure and various combs are released from the main wafer substrate;
step (i) of, to the structure formed through the step (h), etching the silicon etch mask 1(112) by the amount of the thickness of the silicon etch mask 1(112) at the thinner parts within the silicon etch mask 1(112), where the thickness difference of the silicon etch mask 1(112) is caused by the etching of the exposed part of the silicon etch mask 1(112) which are not covered with the silicon etch mask 2(113) at the step (c), and determining the vertical offset between the upper and lower electrodes(119) by performing the fourth silicon etching by an amount of t4; and
step (j) of removing the wall protection layer(114), which has been formed at the step (f), and removing the silicon etch mask 1(112), which is formed at the step (a).